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ETO ETO Exam papers 📅 Apr 2024

Exam Question

(a) What is back emf? Derive the relation for the back emf and the supplied voltage in terms of armature resistance. (6)

(b) The earth-lamps on a main switchboard comprise two 240V 60W lamps connected in the usual manner. The potential difference at the busbars is 220V. Damage by sea water occurs to a distribution cable so that the insulation resistance to the earth is reduced to 16 ohms and 6 ohms for +ve and -ve cables respectively. Find by calculation (10)

(i) Which of the two lamps burns the brightest and

(ii) The additional laod on the generators occasioned by the fault. The resistance of the cables and the ship's structure may be neglected, and that of the lamps taken as constant at the value curresponding to the 60W rating

Exam question diagram

Reference Answer

(a) Back electromotive force (back EMF, Eb​) is the voltage generated in the armature of a DC motor when it rotates and cuts the magnetic flux. By Fleming's Right-Hand Rule , this induced emf opposes the applied voltage V, as per Lenz's law. Back EMF acts as a self-regulating mechanism that limits the armature current when the motor is running.
Consider a shunt motor: 
$$V\:=\:Applied\:voltage$$
$$I\:=\:Current\:flowing\:through\:the\:circuit$$

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